Multi-cavity wavelength tunable laser device

ABSTRACT

A wavelength tunable laser device includes a gain element that provides optical gain. A laser cavity includes at least two cavities, wherein each of the at least two cavities has a different optical cavity length. At least two optical modulators are positioned in the laser cavity, wherein the at least two optical modulators are driven with waveforms so as to tune a wavelength of the wavelength tunable laser device.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.15/717,438, filed on Sep. 27, 2017, entitled “Wavelength Tunable LaserDevice”, which is a continuation of U.S. patent application Ser. No.15/244,503, filed on Aug. 23, 2016, entitled “Integrated OpticalSystem”, now U.S. Pat. No. 10,132,610, which is a continuation of U.S.patent application Ser. No. 14/201,827, filed on Mar. 8, 2014, entitledIntegrated Optical Coherence Tomography Systems and Methods, now U.S.Pat. No. 9,464,883, which claims benefit of U.S. Provisional PatentApplication Ser. No. 61/838,313 filed Jun. 23, 2013. The entire contentsof U.S. patent application Ser. No. 15/717,438, U.S. Pat. Nos.10,132,610, 9,464,883, and U.S. Provisional Patent Application Ser. No.61/838,313 are all herein incorporated by reference.

TECHNICAL FIELD

This disclosure relates generally to the field of optical signalacquisition, processing, and imaging. More particularly, this disclosurepertains to integrated optical coherence tomography systems, structures,and methods and related optical sensing, imaging, and ranging methods,systems and structures employing tunable optical sources and coherentdetection.

BACKGROUND

Optical coherence tomography (OCT) is now known to be a minimallyinvasive optical imaging technique that provides high-resolution,cross-sectional images of tissues and turbid media and which canseamlessly integrates into other diagnostic procedures. OCT can providereal-time images of tissues in situ and can advantageously be used whereconventional excisional biopsy is hazardous or impossible, to reducesampling errors associated with conventional excisional biopsy, or toguide further interventional procedures. Given its exceptional promise,systems and methods for improved OCT, as well as ranging and imagingwould represent a welcome addition to the art.

SUMMARY

An advance in the art is made according to an aspect of the presentdisclosure directed to integrated optical systems, methods and relatedstructures employing tunable optical sources and coherent detectionuseful—for example—in OCT, ranging and imaging systems.

In contrast to contemporary, prior-art OCT systems and structures thatemploy simple, fiber optic or miniature optical bench technology usingsmall optical components positioned on a substrate, systems and methodsaccording to the present disclosure employ one or more photonicintegrated circuits (PICs), use swept-source techniques, and employ awidely tunable optical source(s) and include multiple functions and insome embodiments all the critical complex optical functions arecontained on one photonic integrated circuit.

An illustrative structure according to the present disclosure includesan interferometer that divides a tunable optical signal between areference path and a sample path and combines optical signals returningfrom the reference path and the sample path to generate an interferencesignal, said interferometer including a dual polarization,dual-balanced, in-phase and quadrature (I/Q) detection outputs andintegrated photodetectors and a detection system that detects theinterference signal from which information about a longitudinalreflectivity profile of optical properties of a sample positioned in thesample path may be generated wherein the interferometer and thedetection system are all integrated onto a single photonic integratedcircuit (PIC). The optical information can eventually be represented inthe form of a 1D, 2D, or 3D image. The detection system can be simple(e.g. a transimpedance amplifier (TIA)) or can include more complexelectrical signal processing. The information

Further aspects of this illustrative structure according to an aspect ofthe present disclosure further includes a tunable optical source systemthat generates the tunable optical signal and/or a k-clock module forgenerating a k-clock signal for triggering the detector system whereinthe k-clock, the interferometer, the tunable optical source system andthe detection system are all integrated onto the PIC.

BRIEF DESCRIPTION OF THE DRAWING

A more complete understanding of the present disclosure may be realizedby reference to the accompanying drawings in which:

FIG. 1A shows a schematic diagram illustrating a swept-source OCTconcept;

FIG. 1B illustrates example system specifications;

FIG. 1C illustrates axial and lateral resolution;

FIG. 1D illustrates 1D, 2D, and 3D imaging from a series of axial scansaccording to an aspect of the present disclosure;

FIG. 2 shows a schematic block diagram of a system having a singledual-balanced receiver according to an aspect of the present disclosure;

FIG. 3 shows a schematic block diagram of a system having circulatorsand a 90 degree hybrid exhibiting dual-balanced I and Q channelsaccording to an aspect of the present disclosure;

FIG. 4 shows a schematic block diagram of a system having circulatorsand dual polarization receiver with a polarization splitter and 90degree hybrids exhibiting I and Q channels in two polarizationsaccording to an aspect of the present disclosure;

FIG. 5 shows a schematic block diagram of a system having circulatorsand dual polarization receiver with a polarization splitter and 90degree hybrids exhibiting I and Q channels in two polarizations andsurface grating couplers used to couple light on and off a photonicintegrated circuit (PIC) according to an aspect of the presentdisclosure;

FIG. 6 shows a schematic block diagram of a system having a single PICinput/output port and a dual polarization receiver with a polarizationsplitter and 90 degree hybrids exhibiting I and Q channels in twopolarizations where the system has a laser with long coherence lengthand a delay for the reference arm contained within the single PICaccording to an aspect of the present disclosure;

FIG. 7 shows a schematic block diagram of a system having couplers anddual polarization receiver including a polarization splitter and 90degree hybrids exhibiting I and Q channels in two polarizations and adual polarization modulator according to an aspect of the presentdisclosure;

FIG. 8A shows a schematic block diagram of an embodiment of a k-clocksystem according to an aspect of the present disclosure;

FIG. 8B shows a schematic block diagram of an embodiment of a k-clocksystem according to an aspect of the present disclosure;

FIG. 8C shows a schematic block diagram of an embodiment of a k-clocksystem according to an aspect of the present disclosure;

FIG. 9A shows a schematic block diagrams illustrating a frequencytunable optical source including a ring configuration and opticalfrequency shifter;

FIG. 9B shows a frequency shift including a phase modulator andserrodyne modulation;

FIG. 9C shows a frequency shifter having two Mach Zehnder modulators;

FIG. 10 shows an exemplary output of the frequency shifter of FIG. 9Cconstructed in a PIC according to an aspect of the present disclosure;

FIG. 11A show a schematic block diagrams illustrating a methods forachieving a frequency shift as compared with that shown in FIG. 9C andaccording to an aspect of the present disclosure;

FIG. 11B show a schematic block diagrams illustrating a methods forachieving a frequency shift as compared with that shown in FIG. 9C andaccording to an aspect of the present disclosure;

FIG. 12 shows a schematic block diagram illustrating an exemplaryMach-Zehnder modulator employed as a filter in a tunable laser accordingto an aspect of the present disclosure;

FIG. 13 shows a schematic block diagram illustrating a ring laserconfiguration employing two frequency shifters according to an aspect ofthe present disclosure;

FIG. 14 shows a schematic block diagram illustrating a ring laserconfiguration having a frequency shifter and an optional tracking filterwherein the frequency shifter is configured to shift light entering intoit to a new frequency that is closely aligned with a ring cavity modeaccording to an aspect of the present disclosure;

FIG. 15A shows a schematic block diagram illustrating an embodiment of afrequency shifter wherein a reconfigurable light modulator is employedaccording to an aspect of the present disclosure;

FIG. 15B shows a schematic block diagram illustrating another embodimentof a frequency shifter wherein a reconfigurable light modulator isemployed according to an aspect of the present disclosure;

FIG. 16 shows a schematic block diagram illustrating alternate laserembodiments that employ a linear cavity configuration in closeelectrical communication with one or more DACs to achieve high speedaccording to an aspect of the present disclosure.

FIG. 17A shows a schematic block diagram illustrating a laser embodimentthat does not include a frequency shifter in a cavity;

FIG. 17B shows a schematic block diagram illustrating a laser embodimentthat does not include a frequency shifter in a cavity including a onering resonator configuration;

FIG. 17C shows a schematic block diagram illustrating a laser embodimentthat does not include a frequency shifter in a cavity including a tworing resonator configuration;

FIG. 18A shows a schematic block diagram illustrating a silicon PIChaving an embedded gain chip and surface grating couplers and multiplephase modulators that can act as separate frequency shifters or impartother optical modulation within the laser cavity including one outputsurface grating coupler according to an aspect of the presentdisclosure;

FIG. 18B shows a schematic block diagram illustrating an exampleincluding two output surface grating couplers according to an aspect ofthe present disclosure;

FIG. 19 shows a schematic block diagram illustrating a silicon PIC withan embedded gain chip and end face coupling and a frequency shifteraccording to an aspect of the present disclosure;

FIG. 20 shows a schematic block diagram illustrating a silicon PIC withan embedded gain chip employing end-face coupling and two sets of ringlaser resonators according to an aspect of the present disclosure;

FIG. 21 shows a schematic block diagram illustrating a silicon PIC withan embedded gain chip employing end-face coupling and wherein the laserhas a tunable tracking filter and an arbitrary modulator that can impartphase, frequency, or amplitude modulation on light within the lasercavity according to an aspect of the present disclosure;

FIG. 22 shows a schematic block diagram illustrating a silicon PICsimilar to that shown in FIG. 21 wherein the PIC includes a dualpolarization I/Q modulator such as that shown in FIG. 7;

FIG. 23A shows a schematic block diagram illustrating a tunable lasertransmitter and single polarization I/Q coherent receiver constructed ona PIC;

FIG. 23B shows a photograph illustrating a tunable laser transmitter andsingle polarization I/Q coherent receiver constructed on a single PIC ofFIG. 23A;

FIG. 23C shows a sample output of tunable laser spectrum of the tunablelaser transmitter of FIG. 23A according to an aspect of the presentdisclosure;

FIG. 24 shows a schematic block diagram illustrating a fiber assemblyincluding three single mode optical fibers coupled to three surfacegrating couplers on a silicon PIC according to an aspect of the presentdisclosure;

FIG. 25A shows a schematic block diagram illustrating a PIC packagedwith various electronic components;

FIG. 25B shows a schematic block diagram illustrating a PIC packagedwith various electronic components;

FIG. 25C shows a schematic block diagram illustrating a PIC packagedwith various electronic components; and

FIG. 26 shows a schematic block diagram illustrating an exemplary PICand electronic circuit on a carrier substrate according to an aspect ofthe present disclosure.

DETAILED DESCRIPTION

The following merely illustrates the principles of the disclosure. Itwill thus be appreciated that those skilled in the art will be able todevise various arrangements which, although not explicitly described orshown herein, embody the principles of the disclosure and are includedwithin its spirit and scope. More particularly, while numerous specificdetails are set forth, it is understood that embodiments of thedisclosure may be practiced without these specific details and in otherinstances, well-known circuits, structures and techniques have not beshown in order not to obscure the understanding of this disclosure.

Furthermore, all examples and conditional language recited herein areprincipally intended expressly to be only for pedagogical purposes toaid the reader in understanding the principles of the disclosure and theconcepts contributed by the inventor(s) to furthering the art, and areto be construed as being without limitation to such specifically recitedexamples and conditions.

Moreover, all statements herein reciting principles, aspects, andembodiments of the disclosure, as well as specific examples thereof, areintended to encompass both structural and functional equivalentsthereof. Additionally, it is intended that such equivalents include bothcurrently-known equivalents as well as equivalents developed in thefuture, i.e., any elements developed that perform the same function,regardless of structure.

Thus, for example, it will be appreciated by those skilled in the artthat the diagrams herein represent conceptual views of illustrativestructures embodying the principles of the invention.

In addition, it will be appreciated by those skilled in art that anyflow charts, flow diagrams, state transition diagrams, pseudocode, andthe like represent various processes which may be substantiallyrepresented in computer readable medium and so executed by a computer orprocessor, whether or not such computer or processor is explicitlyshown.

In the claims hereof any element expressed as a means for performing aspecified function is intended to encompass any way of performing thatfunction including, for example, a) a combination of circuit elementswhich performs that function or b) software in any form, including,therefore, firmware, microcode or the like, combined with appropriatecircuitry for executing that software to perform the function. Theinvention as defined by such claims resides in the fact that thefunctionalities provided by the various recited means are combined andbrought together in the manner which the claims call for. Applicant thusregards any means which can provide those functionalities as equivalentas those shown herein. Finally, and unless otherwise explicitlyspecified herein, the drawings are not drawn to scale.

Thus, for example, it will be appreciated by those skilled in the artthat the diagrams herein represent conceptual views of illustrativestructures embodying the principles of the disclosure.

More specifically, much of the discussion that follows is presented withrespect to a swept-source optical coherence tomography system. However,those skilled in the art will readily appreciate that this discussion isbroadly applicable to a wide range of applications that employ on aswept laser (or other optical sources that may be rapidly swept over awide frequency range) and interferometric electro-optical detection, forexample, ranging, medical imaging, non-destructive evaluation andtesting, laser radar, spectroscopy, and communications—among others.

Turning now to FIGS. 1(a)-1(d), and in particular FIG. 1(a), there isshown a schematic of the axial imaging component of an optical coherencetomography arrangement including a swept-source according to an aspectof the present disclosure. More particularly in that FIG. 1(a), afrequency swept light source is coupled to a Michelson interferometerwhich comprises two optical paths or “arms”. Those skilled in the artwill appreciate that while a Michelson interferometer is shown in theillustrative examples described herein other types of interferometersare also possible and are contemplated by this disclosure.

One arm of the Michelson interferometer comprises a reference opticalpath having a mirror which reflects light and the other arm comprises asample optical path into which is positioned a sample whoseaxial/longitudinal reflectivity profile is to be measured.Operationally, light collected from both the reference and sample pathsare interferometrically combined and directed to a photodetector(including subsequent signal processing not specifically shown). Due toa delay between reference and sample reflections, interferometricdetection and frequency sweep of a laser light source, the photodetectoroutput includes information about the axial/longitudinal reflectivityprofile of the sample that may be advantageously extracted by FourierTransform (FT) techniques or other techniques as known in the art. Asmay be readily appreciated, a number of architectures and arrangementsapplying these broad techniques are possible. Exemplary and/orillustrative architectures and arrangements are contemplated andpresented by this disclosure.

More particularly, other types of swept-source, optical coherencetomography (SS-OCT) system topologies that are known in the art arecontemplated by this disclosure. With reference to FIG. 1(b), there areshown exemplary, illustrative specifications for systems constructedaccording to aspects of the present disclosure. In particular, centerwavelength(s)˜1310 nm; Scan Range(s)>100 nm; Coherence Length(s)>20 mm;Sweep Speed(s)>100 kHz; Laser Output Power(s)>25 mW; and Ideal Sweep(s)exhibiting 100% duty cycle sawtooth—are all (as well as others) arecontemplated by this disclosure. Note that these exemplary, illustrativespecifications are in no way limiting. It is understood and thoseskilled in the art will readily appreciate that there are a wide varietyof other specifications contemplated such as different centerwavelength(s), sweep speed(s), etc., contemplated by this disclosure aswell.

With reference to FIG. 1(c), there it depicts in schematic form anaspect of contemporary SS-OCT systems namely, that the longitudinalresolution of such systems is substantially dictated by properties ofthe optical source (i.e., its spectral bandwidth) and the focusingproperties of light onto/into the sample. More particularly, theschematic diagram of FIG. 1(c) shows interrelationships between axialresolution, transverse resolution, depth of field as they relate tosystems exhibiting low numerical aperture focus (NA) and high NA focus.As may be appreciated, for many contemporary OCT systems, it is theoptical spectral bandwidth of the source that is the limiting factor ofits longitudinal resolution.

FIG. 1(d) shows one example in schematic form of how 1D, 2D, 3D imagesmay be constructed by combining axial/longitudinal scanning from a lasersource frequency sweep and Fourier transform processing along withlateral or rotational scanning of light onto a sample via a probe module(not specifically shown) as performed by contemporary systems. One canthen implement lateral, rotational, or transverse scanning to product 2Dand 3D images. Other configurations of SS-OCT systems employing parallelacquisition systems are variations to these contemporary systems.

As previously noted—and in sharp contrast to contemporary, prior-artSS-OCT systems and structures—systems and structures according to thepresent disclosure employ one or more photonic integrated circuits(PICs) that are advantageously constructed using combinations ofoptically compatible material such as Silicon (Si), Indium Phosphide(InP), Gallium Arsenide (GaAs), Indium arsenide (InAs) quantum dots,Germanium (Ge), or other suitable, optically compatible material. Offurther contrast, prior art OCT systems, such as those that do describephotonic integrated circuits, often times do not utilize swept-sourcetechniques, but instead use a very different OCT technology namely,spectral domain optical coherence tomography. And for those prior artsystems that do describe the use of PICs for SS-OCT they do not addressthe integration of many optical functions such as interferometers, dualpolarization, dual balanced, FQ receivers with integratedphoto-detectors and electro-optical integration which is key to makingthis systems robust, manufacturable, small, and low-cost. Finally, priorart OCT systems generally employ simple, miniature optical benchtechnology using small optical components placed on a substrate, and donot include a widely tunable optical source or integrated k-clocks anddetectors.

With these principles in place, we may now examine more particularexemplary configurations and systems according to aspects of the presentdisclosure. Turning now to FIG. 2, there it shows a schematic blockdiagram of an illustrative system having a single, dual-balancedreceiver according to an aspect of the present disclosure. As depictedtherein, a dotted line outlines those components that may beadvantageously incorporated (integrated) into/onto a single PIC as oneexemplary configuration according to the present disclosure.Importantly, and as may be readily appreciated by those skilled in theart, a greater or lesser number of the components shown may beintegrated into the PIC as system design dictates or further benefit(s)arise from such integration. For example in some illustrativeembodiments the transmit laser can be located outside the PIC.

As shown in that FIG. 2, a tunable (transmit) laser is optically coupledto a 90/10 coupler. The 90% output of that coupler is directed to a50/50 coupler, the output of which is directed to a probe module thatcouples light to/from a sample while—in a preferredembodiment—performing a lateral or a rotational scanning of light acrossthe sample. Note that in this schematic FIG. 2, the lateral scanning isnot specifically shown.

Returning to FIG. 2, the 10% output of the 90/10 coupler is directed toan 80/20 coupler that further couples light to and from a referencemodule. As may be readily appreciated, such a reference module mayinclude an fixed or adjustable path length device that sets measurementrange of interest and include other devices (e.g., polarizationrotators, attenuators, lenses, mirrors, etc.) and perform otherfunctions as well. Unused ports of the 50/50 sample path and the 80/20reference path may be terminated or may alternatively be used to supplylight to a k-clock input port.

As shown further in FIG. 2, light reflected back from the 80/20 coupleris shown coupled to a second 90/10 coupler. The 10% output of thatsecond 90/10 coupler is shown coupled to an optional k-clock anddetector—which will be discussed later—and the 90% output of that second90/10 coupler is shown coupled to an optional polarization controllerthat may be automatically or manually adjusted. Notably, it may beadvantageous to include another polarization controller and 90/10coupler in the sample path to balance dispersion and birefringence.

As may be appreciated, the polarization controller so used may be anactive controller that is controlled by the electronics module(connections not specifically shown) or alternatively, be manually set.Reference and sample light are coupled to a 50/50 receiver coupler anddirected to a balanced photo-detector configuration to enhance receiversensitivity and minimize laser intensity noise as well as other noisesources.

Output from the photo-detector is directed into an electrical processingmodule that may advantageously include one or more transimpedanceamplifiers (TIAs), Analog-to-Digital Converter, (ADCs), Digital toAnalog Converter(s), DACs, and Digital Signal Processing (DSP)electronic modules. Advantageously, such electronic modules may beincluded in one or more integrated electronic chips includingApplication Specific Integrated Circuits (ASICs) and/or FieldProgrammable Gate Arrays (FGPAs) as well as other discrete or monolithicelectronic devices. This electrical processing in some embodiments canbe housed in the same electro-mechanical package (co-packaged) or can belocated in a separate electromechanical package.

As shown, the electrical module depicted in FIG. 2 also receives thek-clock output and is further connected to the transmit laser such thatit may control its operational characteristics. Some exemplaryembodiments do not require the use of a k-clock but it is advantageousif the frequency sweep is not linear or highly repeatable as discussedlater. As may be appreciated and understood, sections of the PIC shownin FIG. 2 and external to the dotted line may be optical fibers orfree-space optical links or a combination thereof.

It is worth noting at this point in the discussion that a wide varietyof other coupling ratios and configurations other than those shown arecontemplated and consistent with this disclosure. For example, analternative embodiment may replace the 50/50 and 80/20 output couplers(connecting the sample and references) with circulators such that anincrease in useful signal power and an increase in the isolation ofreflected light with respect to the laser cavity is achieved. Inconfigurations where bulk circulators are used, four externalconnections to the PIC instead of the two shown in FIG. 2 are employed.

Turning now to FIG. 3, there it shows a schematic block diagram of anillustrative system having circulators in place of the 50/50 and 80/20splitters of FIG. 2, and a 90 degree hybrid exhibiting dual-balanced Iand Q channels according to an aspect of the present disclosure. Morespecifically, and as shown in FIG. 3, received optical signals arerouted to a 90 degree hybrid processor that includes four output signalsthat represent two dual-balanced in phase (I_(x)) and quadrature (Q_(x))optical signals. The 90-degree hybrids may be—for example—multimodeinterference couplers, star couplers, or a network of 1×2 and 2×2couplers.

The I (I_(x)) and Q (Q_(x)) signals depicted in FIG. 3 allowphase-sensitive detection of light from the sample and extraction ofadditional optical information on the sample and other signal processingimprovements. Of further advantage, the photodetectors may bemonolithically integrated into the PIC using a suitable optical detectormaterial (e.g., Ge).

In one illustrative embodiment—and as may be readily appreciated bythose skilled in the art—the photodetectors may be butt coupled orotherwise optically coupled to the PIC or located on a separate device.In the illustrative embodiment depicted in FIG. 3, the entire regionshown within dotted lines is advantageously included in/on one singlePIC. In other contemplated embodiments according to the presentdisclosure, the tunable laser may be external to the PIC. Alternatively,only the receiver portion may be included within the PIC, or thecirculators may be replaced with couplers and located within the PIC aswell. As may be appreciated, a great number of configurations arecontemplated by the present disclosure.

FIG. 4, FIG. 5, and FIG. 6 show further illustrative extensions to theseconfigurations shown and described. More particularly, they depictillustrative configurations wherein a received optical signal is routedto a polarization diversity receiver that includes two 90-degree hybridprocessors. Such embodiments advantageously exhibit improvedcapabilities with respect to polarization-diversity andpolarization-sensitivity along with an improved ability to measure boththe sample birefringence and other characteristics along with phasesensitive detection within each polarization. Such phase andpolarization sensitive detection permits functional imaging via Doppler,increased sensitivity and improvements in signal processing and sampleimaging information possibilities such as polarization independentimaging or polarization sensitive imaging.

FIG. 4 shows a schematic block diagram of an illustrative systemincluding circulators and a dual polarization receiver includingpolarization splitters and 90 degree hybrids exhibiting I and Q channelsin two polarizations according to an aspect of the present disclosure.As may be appreciated, such a system may be integrated onto a single PICincluding source(s), k-clock, polarization controller, and dualpolarization receiver.

FIG. 5 shows a schematic block diagram of an illustrative system havingcirculators and dual polarization receiver with a polarization splitterand 90 degree hybrids exhibiting I and Q channels in two polarizationsand surface grating couplers used to couple light on and off a photonicintegrated circuit (PIC) according to an aspect of the presentdisclosure. As depicted therein, the polarization controller isintegrated, however this device is optional and may be included externalto the PIC in the reference arm—or not at all. Alternatively—in thoseenvironments in which the bandwidth is very large—it may be beneficialto have a polarization controller positioned in both the sample andreference paths such as that shown thereby balancing particular opticalproperties such as birefringence, dispersion, and/or other opticalcharacteristics.

With continued reference to FIG. 5, it is noted that phase andpolarization sensitive detection advantageously allows functionalimaging via Doppler, increased sensitivity, and improvements in signalprocessing and sample imaging information capability such aspolarization sensitive detection and imaging.

With continued reference to FIG. 5, it is noted that the illustrativeembodiment shown therein is compatible with surface grating couplers(SGC). As may be appreciated, one dimensional (1D) surface gratingcouplers may be used to direct (couple) light off of the PIC and intocirculator(s) while two dimensional (2D) surface grating couplers may beused to receive reflected light from the circulators and simultaneouslysplit them into nearly orthogonal polarizations. As depicted in FIG. 5,power monitors (PM) are used to monitor polarization alignment and otherconditions of the reference path. Advantageously, wide-band gratingcouplers can be made by using a core material with a lower index thansilicon, such as silicon nitride, and/or by using a smaller spot size onthe grating, for example from a small core fiber. Of further advantage,different combinations of couplers or combinations of 1D and 2D couplersmay be used simultaneously in alternative embodiments.

With reference now to FIG. 6, there it shows an alternative embodimentaccording to an aspect of the present disclosure that particularlyuseful when used with a laser source having sufficiently long coherencelength for the sample measurement distances. As depicted in FIG. 6, theentire reference path length may be located on the PIC. Consequently,only one PIC external connection is employed—the one to the sample. Witha configuration such as that depicted in FIG. 6, an on-chip path delayunit constructed from a tightly wound spiral or other waveguidestructure may be included. To impart low loss and low temperaturedependence such a waveguide structure may be fabricated from SiliconNitride (SiN) or Silicon Oxynitride (SiON) materials.

At this point we note that for improved axial/longitudinal resolution,it is important that each arm exhibit substantially matching totaldispersion and birefringence characteristics. In certain configurationsit is convenient to position/place similar devices in both arms so as tokeep the optical characteristics balanced. For configurations in whichsuch placement of similar structures is impossible or impractical, thenone can—for example—introduce (additional) dispersion into the PICstructure by using—for example—ring resonators (as all-pass filters)coupled to waveguides. Advantageously as an alternative, if the pathcharacteristics are not matched then it is also possible—if thecoherence length of the laser is long and the optical properties arestable—to electronically post process this dispersion or birefringenceimbalance out electronically in the DSP in cases where both I and Qphase sensitive detection is utilized.

Turning now to FIG. 7, there it shows an alternative illustrativeembodiment according to the present disclosure in which a transmitterpath (the output laser light before the probe/sample) includes a dualpolarization modulator and only one output light path from the PIC (thesample and probe module are split external to the PIC). As may beappreciated, the modulator in this embodiment may provide alternatingon/off or other modulation (e.g. within an axial scan or sendingalternate polarizations on adjacent axial scans) into the sample suchthat birefringence information along the axial profile of the sample isextracted. That is to say the modulation may be performedrapidly—relative to a laser frequency sweep time—or may be performedmore slowly, alternatively on each laser sweep or other combinations.

In addition to the functionality described above, the modulator may alsobe used to set arbitrary intensity and phase information on eachpolarization such that the receiver module can perform processing onthis modulation to extract additional features. For example a Hamming orother window can be applied to the laser output amplitude. As discussedearlier it is possible to locate the tunable optical transmit laser (oran equivalently functioning tunable optical source (e.g. and ASE sourceand a tunable filter)) external to the PIC. The polarization combinerafter the two modulators may be either a 2D grating coupler or apolarization rotator and polarization beam combiner connected to a facetcoupler.

At this point we note that polarization splitters, combiners, androtators shown in the various figures are preferably fabricated onto thePIC and exhibit a broad bandwidth, low loss and high extinctioncharacteristics. As those skilled in the art will readily appreciate,there are known a variety of ways to build such individual structuresand devices.

With respect to surface grating couplers, there exist a variety ofdesigns of surface grating couplers—including 1D and 2D grating couplersas well as designs exhibiting various fiber incidence (i.e., normal,slight, extreme)—such that output light is primarily coupled into twooutput waveguides instead of four, for example. As may be appreciated,one advantage of surface grating couplers is they are easy to fabricateand easy to couple light into/out of them. Also surface grating couplerseliminate the need to rotate polarization on the PIC, because bothpolarization (states) signals in the fiber maintain the samepolarization in the PIC. Conversely, one disadvantage of using surfacegrating couplers is that it is difficult to make them such that theyexhibit both a very broad bandwidth a very low loss.

With respect to polarization controller(s) shown in various figures,they too can be implemented in a variety of ways and exhibit a number ofparticular characteristics. By way of non-limiting example(s), it isnoted that a polarization controller needs to exhibit a broad bandwidthand low loss. Also, the polarization controller should not introducesignificant dispersion or birefringence over the laser tuning band. Ifsuch dispersion or birefringence exists then a second matching polarizercan be added—for example—to the sample arm of the system.

Advantageously, “endless” polarization controllers or resettablepolarization controllers may be fabricated within the PIC, using, forexample, a cascade of Mach-Zehnder interferometers. Alternatively, suchpolarization controllers may be located outside or off of the PIC. Whilein some configurations a polarization controller is not needed, in otherconfigurations where it is included it can be set manually, or beelectronically adjustable and advantageously not requiring resets toachieve an arbitrary polarization state (endless polarizationcontroller).

Turning now to FIGS. 8(a)-8(c), there it shows three examples of k-clockprocessing modules according to aspects of the present disclosure. Andwhile three illustrative examples are depicted in FIGS. 8(a)-8(c), thoseskilled in the art will appreciate that additional configurations arepossible and contemplated. Generally, with respect to k-clocks, it isnoted that in some embodiments—such as when a frequency sweep is verylinear in time and repeatable—a k-clock is not needed. In otherembodiments, a k-clock allows one to compensate for non-ideal frequencysweep parameters in the tunable laser. For example if the tunable laseris swept in a sinusoidal (or other waveform) sweep over time, then ak-clock will allow an output clock to be triggered at substantiallyregular frequency increment intervals and such signals will trigger theADC (or be used in alternative digital signal processing if fixed timeADC sampling is used) such that a proper Fourier transform takes place.As discussed previously, structures according to the present disclosureadvantageously integrate k-clock(s) into/onto the PIC along with anumber of other optical and electrical functions.

With initial reference to FIG. 8(a), there it shows a simple dualbalanced embodiment where two 50/50 couplers and a differential pathdelay are used in combination with dual balanced photo detectors.Turning now to FIG. 8(b), there it shows a non-differential embodimentwhere there are two (I and Q) electrical optical and electrical outputsphase shifted by 90 degrees. Finally, FIG. 8(c) shows an illustrativeembodiment where there are two (I and Q) electrical optical andelectrical outputs and each one of them contains a differentialdetection to eliminate common-mode noise. As noted previously, whilethese three illustrative embodiments are shown, those skilled in the artwill understand that other embodiments are contemplated according to thepresent disclosure.

As may be appreciated, it is sometimes beneficial for the optical pathlength—such as that shown previously in FIGS. 1-7—from the laser sourceto the sample and further to the photodetector(s) to have approximatelythe same total delay as the path from the laser source to the triggeringof the ADCs via the k-clock processing module. One way to achieve thisis to have an optical delay between the laser output and the k-clockinput that matches both delay and dispersive properties of the two pathlengths. If the delay is small enough (˜1 cm) then it can be containedwithin the PIC. If the delay is much longer, then a fiber optical patchcord can be designed into the path between the 90/10 coupler and thek-clock input (not shown). Another alternative method to achieve thesame total optical delay is to introduce an electronic delay bufferafter the photo-detection. In particular configurations such as when thelaser souse tuning characteristics permit, such an arrangement may be apreferred one. Finally with reference to FIG. 8(a)-8(c), it is notedthat clock generator and electrical processing elements mayadvantageously comprise TIAs, filters to reduce out of band noise,zero-crossing detectors, AGC elements, digital logic (e.g. OR, XOR)phase shifters, and dummy clocks and other processing functions. Instill another alternate embodiment, the k-clock may comprise a ringresonator filter instead of a Mach-Zehnder interferometer.

As may be appreciated, one critical component of an SS-OCT system—aswell as other optical systems—is the laser source. More particularly, adesirable laser source exhibits the following characteristics namely,rapidly tunable, widely tunable, stable, long-coherence length,desirable optical signal to noise ratio (OSNR), minimal excess intensitynoise, compact, reliable, and inexpensive. It is also advantageous forsuch a laser source to exhibit a near sawtooth waveform in terms ofwavelength (or frequency) vs. time.

FIG. 9(a) shows one illustrative embodiment of a frequency tunablesource according to an aspect of the present disclosure. As shown, thetunable source includes a seed laser, an optical switch, an amplifier, afrequency shifter, a tunable optical filter, an isolator and apolarizer—all configured in a common ring arrangement. Operationally, atthe start of each laser sweep the optical switch is connected to theseed laser. This seed laser provides the necessary output power andcoherence length sufficient to start the frequency sweep whilepreferably saturating the optical amplifier to minimize ASE noise. Theseed laser can be integrated into the PIC or externally located andfiber coupled onto/into the PIC. The light from the seed laser isoptically amplified and sent to the frequency shifter and is maintainedlong enough in the ring to stabilize the light and amplifier.

We note that there exist alternatives to the seed laser such as using asingle frequency reflector in combination with the ring gain element toproduce a laser starting frequency. Also in one embodiment the 2:1switch and seed laser can be eliminated and the tunable optical filteris set to the starting frequency and the frequency shifter is turned offfor a period sufficient for the ring laser to begin lasing on a ringcavity within the tunable optical filter bandwidth.

Notably, the illustrative embodiment depicted in FIG. 9(a) is arrangedas a unidirectional ring. Those skilled in the art will appreciate thatother arrangements using linear cavities or alternative configurationsare contemplated by this disclosure as well. More specifically, the ringarrangement may be fabricated within a single integrated opticalcomponent or particular part(s) of the ring arrangement may be externalto the PIC (e.g. in optical fiber or free space).

Continuing with our operational discussion of the frequency tunablesource depicted in FIG. 9(a), at a particular time (preferably the roundtrip time), the switch is enabled and the light begins to circulatearound the ring as depicted in the line chart. For each circulationaround the ring, the light is shifted in frequency by f_(Δ) and thisshift continues until the desired total sweep range is completed f_(D).The total sweep time is completed in 1/f_(s).

As may be readily appreciated, there are several ways to generate aconstant frequency shift. With reference to FIG. 9(b), there it shows anillustrative example using a simple phase modulator that is reset atinteger multiples of 2π. FIG. 9(c) shows another illustrative example togenerate a constant frequency shift which employs two Mach-Zehndermodulators biased at their null point and driven in their linear range.The two modulators are similar but one input includes a 90 degreeoptical phase shifter. If the Mach-Zehnder modulators are operated intheir linear regime, then the drive signals are sinusoids. If theMach-Zehnders are driven to their full extent of +/−pi, then the drivesignals are triangle waves.

Note that it is important to maintain stable operation of the frequencyshifter and in particular to extinguish any unshifted, spuriousharmonics of the input light. To maintain such conditions, automaticbias control circuits for biasing each modulator at its null positionand adjusting the RF drive amplitude and phase can be implementedsimilar to those used for optical telecommunication systems such asDP-QPSK and other systems that use Mach-Zehnder modulators.

As noted previously, when the frequency tunable source is configured asa ring such as that shown, the ring may optionally include a polarizerto extinguish unwanted light as normally the ring runs in a singlepolarization. The optical gain may be provided by rare-earth (e.g.Yb/Er) doped waveguides; from monolithically integrated optical gainelements like InP, GaAs, Germanium, or III-V quantum dot material suchas InAs; or using a wafer bonded or butt coupled optical gain elementssuch as InP or GaAs, or other semiconductor material either integratedwith the PIC or external to a PIC that can be optically or electricallypumped. Similarly, frequency shifters may be fabricated in any of anumber of optical compatible materials. Notably, if they are fabricatedin Si, they may be either carrier injection or carrier depletion typemodulators—or even both if they are modulators having an oxide in thejunction.

Note that ring structure depicted in FIG. 9(a) includes an opticalisolator and a tunable optical filter. In an alternate embodiment(s) theisolator and/or tunable optical filter is/are not needed. This isparticularly appropriate in those situations wherein the frequencyshifter produces minimal carrier leak through and spurious harmonics andthe number of circulations of the ring over a scan period issufficiently small such that only modest amplified spontaneous emission(ASE) and harmonic noise builds up in the ring.

With further reference to FIG. 9(a), it is noted that a tunable opticalfilter is employed. This tunable optical filter is not always needed butfor certain configurations, such as a large number of lightcirculations, it can be beneficial. Advantageously, it may be fabricatedusing a narrow filter with fine tracking or relatively coarse bandwidthfilter with corresponding coarse tracking. One advantage of the tunablefilter is that it can suppress any residual ASE noise and spurioussignals (such as unshifted light leaking through the frequency shifter)from building up in the laser cavity.

With this additional suppression of the tunable optical filter, thenumber of cycles of the loop can be increased and the cavity lengthdecreased to the point where the entire laser is housed in a PIC.Additionally, a polarizer can be employed to eliminate unwanted ASE andlight scattered into the orthogonal polarization (as indicated in FIG.9(a)). Notwithstanding, it is possible to use a gain element thatsupports one polarization and thus there is no need for an additionalpolarizer. Furthermore, an optical isolator may be incorporated toensure unidirectional operation if needed. This isolator can be off thePIC or can be contained on the PIC. Also as mentioned above and shown inFIG. 9(a), an alternative to the use of the optical switch is to use afiber optic coupler and two on/off switches. Such an approach can beeasier to control and implement at the expense of increased throughputloss.

Shown further in FIG. 9(a) is a circuit for controlling the optical gainor output power of the optical amplifier (OA). For example, inconstant-power mode, a small optical tap at the output of the OA is usedto estimate the output power and this signal is fed back to internalparts of the OA (e.g. pump or VOA) to keep the output power constant.Other methods for power and gain control are known and contemplated aswell.

Finally with reference to FIG. 9(a), note that a constant frequencyshift is illustrated in this figure. Notwithstanding, it is possible toadjust the frequency shift over time to account for slight variations inpropagation delay around the loop with wavelength if desired.

FIG. 10 shows a graph of our measured example of the frequency shiftershown in FIG. 9(c) in operation using a Si photonic I-Q modulator. Thisfrequency shifter was created in a silicon photonic integrated circuit.The line shown in the center of the graph is some of the carrier leakthrough. The frequency shift is −15 GHz and the side mode suppressionratio (SMSR) is 15 dB. As may be appreciated, the performance of thisfrequency shifter may be improved with better electrical tuning howeverthe figure clearly shows the functionality of the device.

With reference now to FIGS. 11(a) and 11(b), there it shows twoadditional embodiments of frequency shifter topologies for homodynefrequency tracking in telecommunication systems. With reference to FIG.11(a), the embodiment shown is a single side band (SSB) modulator. Itcomprises an I-Q modulator driven by two triangular waves havingpeak-to-peak amplitude of π and a 90° relative phase shift.Operationally, the output rotates endlessly around the origin of thecomplex plane with a linear change in phase with time.

As is known, SSB modulation is traditionally performed at a fixedfrequency. One advantage of the embodiment of FIG. 11(a) is themodulator is driven from a digital-to-analog converter (DAC) with alook-up table, and the driving frequency constantly varies as the tableis read out at a speed proportional to the required phase shift rate.Notably, SSB modulators are traditionally made in LiNbO3 and designed torun at high speeds. In this application however, we wish to operate theSSB modulators at low speeds (<1 GHz) and in a silicon PIC.Advantageously, this allows us to use current injection rather thancarrier depletion modulation, resulting in low optical loss and lowdrive voltages. Finally, an SSB modulator such as that depicted in FIG.11(a), exhibits a 6-dB excess loss.

FIG. 11(b) on the other hand, shows a schematic SSB modulator designexhibiting only 3-dB excess loss. This design has the additionaladvantage of requiring only sinusoidal drive signals rather thantriangular wave signals, which may be easier to generate. The design ofFIG. 11(b) exhibits lower loss because rather than using just 3-dBcouplers on the input and output it uses Mach-Zehnder switches thatredirect the distribution of light between the I and Q modulators as thephase progresses around its circle. These are driven with twice thefrequency as the I and Q modulators but only ˜ 1/13 of the amplitude.

FIG. 12 shows one of many possible examples of a tunable filter that canbe driven in a sinusoidal or preferably a digital fashion to minimizedegradation in the output laser quality due to build up of ASE and/orspurious harmonics in the laser loop. (Note that in this disclosure wemay refer to this loop as a laser loop even though in some embodimentsthere is not laser action and the cavity operates more like a longtransmission line). The basic concept is that of a Mach-Zehndermodulator with unequal path lengths. The path length difference ischosen to be such that the transfer function has a transmission null atone f_(Δ) above and below the desired frequency shifted carrier and atall the subsequent periodic nature of the transfer function. These arequalitatively illustrated by the “X's” on FIG. 12. On the even trips one(blue) transfer function is applied. On the odd cavity trips the other(red) transfer function is applied. It is important that the modulatorbe driven rapidly so as to avoid collapsing the signal amplitude due torepeatedly passing through the modulator (similar to “eye closure” indigital communication signals). This modulator can have automatic biascircuitry that control the bias point (at null) and controls therelative amplitude and phase of the RF square wave or sine wave drivesignal. Note the filter of FIG. 12 works well when driven in a widebandwidth square wave fashion. And while it is also possible to drivethe modulator with a sine wave which need not have a broad bandwidth,this can result in more distortion of the ideally constant amplitudesignal but has the advantage of easier RF drive requirements.

To address the fact that the periodic power transfer function of theMach-Zehnder filter may not ideally follow the constant frequencyincrement of the laser over a 100 nm or more sweep it is possible toutilize a more intelligent waveform than a simple sine or square wave toaccount for keeping the carrier at the center of one of the periodicpeaks of the Mach-Zehnder transfer function at all times.Advantageously, it is also possible to alter the frequency of thefrequency shifter.

Advantageously, it is possible to use more than one stage ofMach-Zehnder filtering. There are a variety of modulator delayconfigurations that can be used and the basic concept is to place nullsof each stage to eliminate spurious leak though of the carrier andunwanted harmonics. In many applications it suffices to have one stage.In other applications where a large number of cavity sweeps is desiredtwo or more stages can be used. In order to drive the Mach-Zehndermodulator properly one approach is to provide a high-speed multi-channelDACs closely coupled to the Mach-Zehnder modulators. To keep theMach-Zehnder path lengths short and within a single PIC it is beneficialto include a high frequency shift in the ring (e.g. 10 GHz).

Note that gain sections for tunable optical sources according to thepresent disclosure may comprise semiconductor optical amplifiers (SOAs),doped waveguide amplifiers, wafer bonded gain elements on siliconwafers, InP re-growth, germanium doped silicon lasers, or doped fiberamplifiers. It is also possible to configure multiple gain sections inparallel using WDM or other splitting/combining techniques to broadenthe bandwidth. That is to say one could use multiple SOAs (or other gainmediums) in parallel connected in phase and with equal path lengths butdifferent gain spectrum peaks.

In another alternative tunable laser embodiment according to the presentdisclosure are one(s) in which there are two or more frequency shiftersin the laser cavity as, for example, shown in FIG. 13. Such anembodiment operates analogous to a Vernier laser cavity in which thelight is split between two resonant cavities with differentfree-spectral ranges. Only certain cavity modes line up in both cavitiessimultaneously acting like an intracavity filter.

In one embodiment of this the rate of change of the frequency shifter isless than the cavity round trip time. In this structure is a laserundergoing laser oscillation and so the frequency sweep could be as slowor fast (the sweeping in one preferred embodiment is slow compared tothe round-trip time) and there is much less concern about degradation inthe buildup of amplified spontaneous emission (ASE) noise and subsequentreduction in optical signal to noise ratio (OSNR) of this approach dueto its laser cavity characteristics than other approaches. The sweep inthis embodiment could be continuous or it could be stepped. DACs (notspecifically shown) can be used to directly drive the frequencyshifters.

One interesting aspect of this embodiment is that these I-Q modulatortypes of frequency shifters are fundamentally different thanacousto-optic frequency shifters in that the laser light adiabaticallyjumps back cavity modes as the laser is swept in frequency.Advantageously, seed lasers, tunable optical filters, isolators andother elements can be added to this cavity to improve operation at theexpense of complexity. FIG. 13 shows two frequency shifters in parallelhowever, more or less could be used. More frequency shifters configuredin parallel can produce a better rejection of unwanted cavity modes andthe expense of increased complexity and wafer yield issues.

FIG. 14 shows yet another embodiment of a tunable optical sourceaccording to an aspect of the present disclosure. More particularly,FIG. 14 shows a schematic block diagrams illustrating a ring laserconfiguration having a frequency shifter and an optional tracking filterwherein the frequency shifter is configured to shift light entering intoit to a new frequency that is closely aligned with a ring cavity mode.In this embodiment the frequency shifter is driven at a rateapproximately equal to the round trip frequency (1/(round-trip-time)) ora multiple of the round trip frequency. In this fashion the lightremains more coherent as it propagates around the cavity, and therebysuppresses ASE noise buildup and at each circulation the frequency isincremented and this results in a step wise sawtooth frequency sweep. Anoptional isolator can be included in the cavity but in a number ofembodiments it is not needed.

The laser depicted in FIG. 14 may benefit from a tunable tracking filterto extend the tuning range and to keep unwanted cavity modes frombuilding up in power. In other embodiments—particularly where smalleroptical frequency sweep is needed—no tracking filter is needed.

Shown as an illustrative example, in the lower right hand corner of FIG.14 there is the optical amplifier gain spectrum, the ring cavity modes,and the tracking filter. In one embodiment the tracking filter has afinesse of ˜100, a bandwidth of ˜1 nm, and a tuning speed of 10 MHz.There are a variety of other embodiments that are possible. Note that itmay be highly beneficial that the tracking filter be synchronous withthe cavity mode hoping from mode to mode.

As is known, chromatic dispersion and non-linear tuning of the filtercan cause it to become slightly misaligned. It is possible to adjust thefrequency shifter drive frequency and/or the tuning rate of the filterso they remain properly aligned. The seed laser can be connected by anon/off switch and a coupler, a 2:1 optical switch, or the seed laseritself can be turned on or off directly. Such a laser can be aligned toone of the lower cavity modes and have the proper power and coherentlength characteristics suitable for the imaging application. The lasercan be directly turn on/off at the start of the sweep or the laser canbe left on to achieve stable operation and a separate on/off modulatorcan be use.

As noted above, to account for slight changes in the round-trip-time asthe laser is scanned in frequency the frequency shifter frequency can beslightly adjusted in time to ensure that the shift of the light remainsat or near a cavity resonance mode.

If the frequency shifter depicted in FIG. 14 is constructed using theapproach described previously with respect to the configuration of FIG.9(c), then it is possible to alter the function of the device “on thefly”. That is, the device of FIG. 9(c)—with the addition of the normalelectrically adjustable phase trims (not specifically shown)—canadvantageously implement a variety of intensity and phase modulationwaveforms.

For example it is possible to slowly or rapidly change the modulationformat from “pass through mode” (e.g. no intensity or phase modulation)to frequency shifting mode. One advantage of this type of operation isit is possible to let the laser light circulate more than oneround-trip-time within the laser cavity. This has the further advantageof allowing the sweep rate to be decreased (for more SNR during datacollection) and allows the laser light to increase its coherence andsettle for a longer time into the proper laser cavity mode. Oneadditional benefit of this approach is that the laser sweep rate can bereconfigured on the fly to integer multiples of the fundamental sweeprate.

A basic idea behind this operation is that the laser operates at acavity mode for one or more round-trip times. Then it is desired to moveto a new cavity mode. Instead of just tuning a filter and restarting thelaser at the new cavity mode and waiting for light in the laser cavityto build up from ASE and other noise sources, the new laser cavity modeis seeded with a strong light signal from the previous cavity mode. Thishas the benefit of improving both the coherence of the light and therate at which the laser cavity can be tuned.

Note that with configurations such as those depicted in FIG. 14 thereare various types of tracking filter that can be used including a singleor multiple set of coupled ring resonators, Mach-Zehnder, Fabry Perot,and grating filters. As noted, it is possible to use no tracking filterat all specially for relatively short sweep ranges.

With reference now to FIGS. 15(a) and 15(b), there they show alternateembodiments of a reconfigurable laser modulator that exhibits a state ofeither frequency shift or pass-through. The top path in each FIGS. 15(a)and 15(b) includes a frequency shifter. The bottom path includes a pathlength and loss trimming and matching fiber and other electro-opticalcharacteristics that can be matched to the upper path. FIG. 15(a) showsan example configuration wherein on/off modulators are used along withpassive couplers. FIG. 15(b) shows an example wherein 1:2 and 2:1optical switches are used. Such switches can be Mach Zehnder or othertypes of integrated optical switches.

At this point it is notable that it may be beneficial to use siliconphotonics for much of the PIC fabrication and couple another type ofelectrically or optical pumped optical gain medium that is configured towork in a double pass geometry through a gain medium. A double pass gaingeometry can be beneficial in embodiments where the majority of the PICis a single silicon photonic integrated circuit and that PIC is buttcoupled (or otherwise coupled) to an InP or other material optical gainmedium. It is possible to use a beam splitter and a double passamplifier (where one facet of the amplifier is HR coated) instead of aunidirectional amplifier. Another approach is to use a combination ofhalf wave plate and quarter wave plates and a polarization beam splitterto allow for more efficient operation. However such polarizationisolation approaches require the gain medium be able to support bothpolarizations.

FIG. 16 shows an example of an embodiment of a tunable laser wherein thefrequency shifter operates in a linear cavity (not a ring configuration)and a gain medium operates in a double pass geometry. High speed DACscan be directly coupled with the tuning elements to ensure rapid,high-speed agile tuning.

With reference now to FIG. 17(a), there it shows linear cavity examplesof tunable lasers that do not employ a frequency shifter modulator. Whenthere is no frequency shifter, in one exemplary embodiment the lasermode hops as it tunes. In other alternative exemplary embodiments cavitylength adjustments may be made along with other approaches to minimizemode-hopes. It yet another exemplary embodiment the laser operates inseveral laser modes at once (e.g. multi-mode) and the groups ofdifferent modes are active as the nominal laser frequency is tuned.

FIG. 17(b) shows an exemplary embodiment employing one ring resonatorand FIG. 17(c) shows another exemplary embodiment where two ringresonators are employed. In a preferred embodiment of the embodiment ofFIG. 17(c), the ring resonators have different free spectral ranges andthe two rings operate in a vernier tuning mode to extend the tuningrange of the laser beyond that possible with just one ring resonator.

FIGS. 18(a)-18(b) shows an example of a complete photonic integratedcircuit similar to some of the systems shown in previously. The receiverportion is a dual polarization I/Q dual balanced configuration similarto those shown in FIGS. 4, 5, and 6. This particular illustrativeembodiment uses a silicon photonic PIC with a recessed region thatcontains an InP two-channel optical gain element. One side of the InPgain element contains HR coatings and the other side (which interfaceswith the SiPh PIC) contains angled facets to minimize any unwantedreflections. As described earlier an alternative to this butt coupledInP gain element approach shown in FIG. 18, other types of but coupledgain elements other than InP can be used and furthermore it is possibleto monolithically integrated the gain on the PIC substrate (and not usebutt coupling) by using known approaches such as growth of III-V quantumdots (e.g. InAs), Germanium, or InP or by using wafer bonding approachesand evanescent or other optical coupling of the light from the siliconphotonic circuit into the bonded optical gain element. The optical gainelements can be optically or electrically pumped.

In this embodiment there are two separate gain elements in the InP chipthat contain gain peaks at different wavelengths. In this manner it ispossible to have an optical frequency sweep that is broader than onegain element can provide. In another embodiment (not shown) one elementis used instead of two for simplicity. In other embodiments there couldbe more than two gain elements for even broader frequency sweeping. Atthe output of the upper gain element there is a phase shifter. Thisphase shifter can be thermal or electro-optically tuned.

One purpose of this phase shifter is to match the nominal optical pathlengths such that in spectral areas where the laser light hassignificant components from both gain elements the light from each gainelement constructively combines in the coupler. One purpose of the MachZehnder combiner (M/Z Combiner) is to optimize coupling of light to theupper gain element or the lower gain element. For example when the laseris operating at a wavelength aligned with the peak of the lower gainelement this M/Z would have a null at the upper gain element gain peak.At a laser wavelength aligned with the peak of the upper gain elementthe M/Z would have a null in transmission at the lower gain peak. ThisM/Z could also contain adjustable phase shifter elements (not shown) toallow for active alignment. There are other combinations of M/Zfiltering functions and gain peak arrangements that are possible.

Operationally, the laser depicted in FIGS. 18(a)-18(b) operates in amanner similar to that shown in FIG. 13 in that two frequency shiftersare utilized and driven at different rates but the laser is in aMichelson interferometer embodiment instead of a ring cavity laserconfiguration. There are two frequency shifters and the outputs of thefrequency shifters are connected to loop mirrors and thus the frequencyshifters operate in a double pass configuration. Note that this laserembodiment could be replaced with the other laser embodiments asdescribed elsewhere in this document. This includes a single frequencyshifter approach, a single frequency shifter with a tunable trackingfilter, and a tunable laser with no frequency shifter at all. One couldemploy more or less frequency shifters than shown in FIGS. 18(a)-18(b).

Note that while FIGS. 18(a)-18(b) do not explicitly illustrate a seedlaser, those skilled in the art will recognize that an integrated orexternal seed laser with appropriate interconnect may be incorporatedinto the structure(s) shown therein according to aspects of the presentdisclosure. Alternatively, and as discussed previously, there are waysto eliminate the need for a seed laser by incorporating wavelengthselective optical elements.

The PIC output couplers and PIC input couplers are surface gratingcouplers and may be similar to those shown previously in FIG. 5 exceptthat in FIG. 18(a), only one PIC output surface grating coupler (SGC)port is used and the reference and sample probe light splitting is doneexternal to the PIC. FIG. 18(b) shows an alternate embodiment for theProbe out and the Ref out that has a 90/10 splitter and two 1D outputgrating couplers on the PIC.

The reference input coupler is a 1D surface grating coupler and leads totwo multi-mode interference (MMI) couplers to provide for X and Ypolarization. The probe input coincides of a 2D surface grating couplerwith normal fiber incidence. Each of the two common polarization armsare couple via a phase shifter and nearly 50/50 coupler into a commonoptical path and then coupled to the MMI couplers. The output of eachMIMI coupler consists of two differential outputs that form adual-balanced I/Q receiver. The unused ports of the near 50/50 couplerscan be used for power monitoring. An alternative to using a 2D normalincidence surface grating coupler is to use a 2D non-normal incidencecoupler.

With reference now to FIG. 19, there it shows another illustrativeembodiment according to the present disclosure that uses facet couplersinstead of the surface grating couplers shown in FIG. 18(a). One benefitof facet couplers is they can achieve both low loss and very broadcoupling bandwidth at the expense of fabrication and alignmentcomplexity and the requirement for a planar polarization splitter androtator. To achieve polarization rotation and splitting then the facetcouplers are followed by integrated polarization beam splitters andintegrated polarization rotators (PBSR) in the probe arm input channel.To achieve a long delay with low loss in the long arm of the k-clock(e.g. 2-20 mm) SiN, SiON, or other waveguide structures can be used.Also shown in FIG. 19 is a seed laser that contains a fixed (or tunable)Bragg grating reflector. This seed laser and be turned on and off byapplying electrical current to the gain medium. The seed laser can beused to start the initial conditions of the frequency sweep in the lasercavity. The seed laser is optional.

For very broad coupling bandwidth, one can use facet couplers withspot-size converters as shown in FIGS. 19, 20, and 21. If these facetcouplers are used instead of the 2D grating couplers, then the facetcouplers must be followed by integrated polarization beam splitters andintegrated polarization rotators in one output of the polarization beamsplitters (PBSR).

An integrated polarization beam splitter can be, for example, adirection coupler in silicon wire waveguides that is 100/0 coupling forTM and nearly 0/100 for TE. A polarization rotator can be, for example,an adiabatic transformation that uses asymmetric waveguidestructures/placements to achieve significant mode splitting when thewaveguide modes are hybrid TE/TM modes.

Note in both FIGS. 18(a)-18(b) and 19 it is possible to configure usinganother embodiment that only has one frequency shifter in series with atunable optical filter, or no frequency shifter at all and just tunableoptical filters. It is also possible to add in PIC optical isolatorsusing couplers and phase modulators.

FIG. 20 shows another illustrative embodiment integrated onto a SiPh PICincluding with a widely tunable laser using tunable filters. As may beobserved, a Mach-Zehnder interferometer (MZI) switch switches betweentwo ring-resonator-based tunable filters. The ring resonators areVernier tuned.

The tuning works as follows. The MZI sends the light to the uppertunable filter. The upper filter begins to tune from one end of the gainspectrum to the other. When the phase tuners in the rings run out ofadjustment range, the second filter is adjusted to be at the samewavelength as the upper filter and same phase but using phase tuners setat the beginning of their ranges. The switch then switches and the lowerfilter tunes and the phase tuners in the upper filter reset.

When the lower filter exceeds its adjustment range, the switch switchesback to the upper filter and the overall process continues. As may beappreciated, this type of swept laser may experience mode hops as thewavelength is tuned. However, phase tuners may be positioned in eachring resonator section such that they remain in a cavity mode and theswitch operates every time one of these phase shifters exceeds itsnormal range. In this way the frequency sweeping could be mode-hop freeor with reduced mode-hops. In order to be near mode-hop free, as theswitch switches, the relative phase between the two paths is adjusted bezero, so that during the switching, which necessarily takes a finiteamount of time, the laser does not mode hop. Also, other tunable filterscould be substituted for these double-ring resonator structures.

Alternatively, if one does not care about the presence of mode hoppingduring tuning, then one could eliminate the switch and just oneVernier-tuned ring resonator set. In this case, one possibility is todrive the two ring resonators with programmed voltages viadigital-to-analog converters so that the wavelength sweep is monotonicacross the band. There would likely be mode hopping because the ringvoltages would have to be non-monotonic and would have to reset attimes. An alternative possibility is to drive one ring with a monotonicvoltage waveform, leaving the other one substantially constant. Thiswould cause the wavelength to tune in discrete steps. After this sweepof one ring, then the second ring could be adjusted a small amount andthen the first ring swept again. This would allow one to eventuallycover all the wavelengths in the band, but in a non-monotonic,moving-comb fashion. Post detection reordering of the frequency samplesin a DSP unit could be used to perform the FFT.

In yet another illustrative embodiment of the structures depicted inFIG. 20 it is possible to use just one gain element in the gain chipthereby eliminating the M/Z combiner and reducing fabrication complexityat the expense of tuning range. It is also possible to reducefabrication complexity and cost to use just one set of tunable filtersand thus eliminate the M/Z switch in applications that require lesstuning speed and tuning range.

Yet another illustrative embodiment according to the present disclosureis shown schematically in FIG. 21. In this illustrative embodiment shownin FIG. 21 there is a single frequency shifter and a tunable trackingfilter constructed using a length-imbalanced Mach Zehnder interferometerhaving a large free-spectral range. As may be readily appreciated, alarge free-spectral range makes tracking easier. A narrower band tunabletracking filter can be used but requires a more complicated filterstructure if it is desirable to tune the whole frequency band withoutany resets. Also there is one output fact coupler/spot-size converterfor the probe and reference outputs which are then split using anexternal splitter.

Yet another illustrative embodiment according to the present disclosureis shown schematically in FIG. 22. It contrast to the structuresdepicted in FIG. 21, the structure(s) of FIG. 22 includes adual-polarization arbitrary I/Q modulator comprising out of phaseshifters, splitters, combiners, and a Mach-Zehnder modulator. Thevarious phase shifters are for adjusting optical path lengths and may becarrier depletion, or other types of modulators and may be thermal orelectro-optically activated. The outputs of the two modulators arecombined in a PBSR and it is also possible to use simpler absorptivetypes of modulators at the expense of higher loss. Other types ofmodulators are possible such as fast VOAs.

FIG. 23(a) shows an illustrative example schematic of silicon PICaccording to an aspect of the present disclosure. A receiver portioncomprises a single-polarization dual-balanced I/Q receiver similar tothat shown previously in FIG. 3. The PIC delay is included within thePIC. A laser contains InP gain chip butt coupled to a silicon photonicintegrated circuit similar to that shown in FIGS. 18-22. The lasercavity includes of two Vernier tuned ring resonators, a fast phase tunerelement, and a single loop reflector. The output waveguides are coupledto a facet coupler labeled “output” which further coupled to a singlemode optical fiber. FIG. 23(b) shows a photograph of the device. FIG.23(c) shows the output laser tuning characteristic over ˜4.8 THz. Widerwavelength tuning is possible.

Note that the structures depicted in FIGS. 18-23 show an optical gainchip set into a silicon photonics PIC. There are a variety of othermethods to add an optical gain compatibility with a silicon substratesuch as using wafer bonding, regrowth, or directly doping the siliconPIC with germanium or rare earth dopants to provide gain. Furthermore itis possible to build the entire PIC out of another optically compatiblemedium such as InP, InAs, GaAs, GaAlAs, InGaAs, or many other opticallycompatible semiconductor materials. For example, it has beendemonstrated that InAs quantum dot (QD) lasers can be applied directlyto silicon to produce optical gain in the 1.3 um region. Some of thesecited approaches can have the benefit of providing gain in one mediumbut the disadvantage of being less compatible with the silicon processescommonly used in semiconductor foundries.

To couple from a PIC to either a fiber or free space optics, a broadbandlow-loss coupling is needed. As discussed earlier, two common methods toachieve this are surface grating couplers and fact coupling (alsoreferred to as end-coupling or butt-coupling). Such coupling is neededat the interfaces from the integrated components (dotted lines in FIGS.2-7 and in the Probe out, Ref Out, Probe In, and Ref In of FIGS. 18-23)or wherever there are input/output locations where light travels on oroff the PIC.

Coupling may also be needed—as discussed earlier—if the swept sourcelaser contains optical path lengths in fiber, and/or if the increaseddelay is needed between the 9/10 coupler and the k-clock input. Asdiscussed earlier in some particular embodiments PIC surface gratingcouplers are used and in other embodiments facet/end/butt coupling isused. To achieve a robust and manufacturable system, it is convenient toplace multiple fibers (2, 3, 4, or more depending on the systemrequirements) in a single glass block that is precisely manufactured tohave the same dimensional separation between fibers as the separation ofthe PIC inputs and outputs. The fibers can be housed and secured in theglass block using epoxy and polished as a unit to ensure low-losscoupling. A manual or automatic multi-axis machine can be used to alignthe glass block to the fiber waveguide interfaces on the PIC. FIG. 24shows and example of a low loss fiber assembly housing three single modeoptical fibers coupled to a silicon photonic circuit containingmodulators and receiver that we have constructed.

A PIC may be housed our otherwise contained in any of a number ofoptical mechanical packages known in the art. However it is highlybeneficial if the PIC is closely integrated with the transimpedanceamplifiers (TIA) and that both are contained in one package. There areseveral methods for achieving this proximity as shown in FIGS.25(a)-25(c) which depict co-packaging of the PIC and electronics. FIG.25(a) shows the PIC mounted in a ceramic or metal package, wirebondedwith TIAs and driver circuits. The driver circuit may contain modulatordrivers, phase shifter drivers, thermal drivers, and DACs, among othercomponents.

Alternatively those active electrical components may be located externalto the package. FIG. 25(b) shows an example where the PIC is co-packagedwith the TIAs and a digital circuit such as an ASIC, FPGA, or othermixed signal electronics.

FIG. 25(c) shows an example embodiment in which the TIAs are furtherintegrated with an application specific integrated circuit (ASIC). Wirebonds, die bonds, wafer stacking, and other approaches can be used tooptically, mechanically, and electrically interface with the PIC.

FIG. 26 shows an illustrative example where the PIC and ASIC are diebonded to a substrate that may comprise silicon, FR4, or anothersuitable substrate carrier that also contains ball bonds. It is possibleto replace the substrate ball bonds with leads or pins in alternateembodiments. The substrate carrier could be active or passive device.Also shown is a metal cover and heat sink and thermal coupler to connectthe top of the ASIC to the cover and heat sink.

At this point those skilled in the art will readily appreciate thatwhile the methods, techniques and structures according to the presentdisclosure have been described with respect to particularimplementations and/or embodiments, those skilled in the art willrecognize that the disclosure is not so limited. In particular, wheremultiple integrated chips are employed, those chips may advantageouslybe closely coupled by positioning them on a common carrier or within acommon packaging. As may be appreciated, in this manner the chips may bephysically close to one another of close in time to one another asappropriate. Accordingly, the scope of the disclosure should only belimited by the claims appended hereto.

The invention claimed is:
 1. A wavelength tunable laser devicecomprising: a) a gain element that provides optical gain; b) a lasercavity comprising at least two optical cavities, wherein each of the atleast two optical cavities has a different optical cavity length; and c)at least two optical modulators, one of the at least two opticalmodulators being positioned in one of the at least two optical cavitiesand the other of the at least two optical modulators being positioned inthe other of the at least two optical cavities, wherein the at least twooptical modulators are each driven with waveforms so as to tune awavelength of the wavelength tunable laser device.
 2. The wavelengthtunable laser device of claim 1 wherein at least one of the at least twooptical cavities comprises a ring cavity.
 3. The wavelength tunablelaser device of claim 1 wherein at least one of the at least two opticalcavities comprise a linear cavity.
 4. The wavelength tunable laserdevice of claim 1 wherein at least one of the at least two opticalmodulators comprises a phase modulator that is driven around an originof a complex plane.
 5. The wavelength tunable laser device of claim 4wherein the phase modulator imparts a linear change in phase as afunction of time.
 6. The wavelength tunable laser device of claim 1wherein the wavelength tunable laser device is formed on a photonicintegrated circuit (PIC).
 7. The wavelength tunable laser device ofclaim 1 further comprising an SS-OCT measurement system wherein anoutput of the wavelength tunable laser device is optically connected toa source input of the SS-OCT measurement system.
 8. The wavelengthtunable laser device of claim 1 wherein at least one of the at least twooptical modulators comprises a phase modulator.
 9. The wavelengthtunable laser device of claim 1 wherein the laser cavity comprises atleast three optical cavities, each of the at least three opticalcavities having a different optical cavity length.
 10. The wavelengthtunable laser device of claim 9 wherein the at least three cavitiescomprise at least three optical modulators, a corresponding one of theat least three modulators being positioned in a corresponding one of theat least three cavities.
 11. A frequency tunable laser comprising: a) afirst laser cavity configured to have a first plurality of cavity modes;b) a second laser cavity optically coupled to the first laser cavity,the second laser cavity configured to have a second plurality of cavitymodes, wherein an optical cavity length of the first laser cavity isdifferent from an optical cavity length of the second laser cavity; c)an optical gain element positioned in a common section of the coupledfirst and second laser cavities, the optical gain element providingoptical gain in the first and second laser cavities; d) an opticalsplitter comprising a port optically coupled to the common section ofthe first and second laser cavities, and a port that couples laser lightto the first and second laser cavities; e) a first phase modulatoroptical coupled to the first laser cavity and configured to shiftoptical phase of light in the first laser cavity as a function of time;and f) a second phase modulator optically coupled to the second lasercavity and configured to shift optical phase of light in second lasercavity as a function of time, wherein modulation provided by the firstand second phase modulators cause tuning a frequency of the laser lightas a function of time.
 12. The frequency tunable laser of claim 11wherein the first laser cavity is configured as a ring cavity.
 13. Thefrequency tunable laser of claim 11 wherein the first laser cavity isconfigured as a linear cavity.
 14. The frequency tunable laser of claim11 wherein the first and second laser cavities, the optical gainelement, and the first and second phase modulators are formed on one ormore closely coupled photonic integrated circuits (PICs).
 15. Thefrequency tunable laser of claim 14 wherein at least one of the firstand second phase modulator is fabricated in silicon material.
 16. Thefrequency tunable laser of claim 15 wherein at least one of the firstand second phase modulator comprises a carrier-depletion modulator. 17.The frequency tunable laser of claim 15 wherein at least one of thefirst and second phase modulator comprises a carrier injectionmodulator.
 18. The frequency tunable laser of claim 14 wherein theoptical gain element is fabricated in InP material.
 19. The frequencytunable laser of claim 14 wherein the photonic integrated circuitcomprises at least two different materials.
 20. The frequency tunablelaser of claim 19 wherein the optical gain element is formed of a firstmaterial and at least one of the first and second phase modulator isformed of a second material.
 21. The frequency tunable laser of claim 14wherein at least one of the first and second phase modulator isconfigured to operate in a double pass configuration.
 22. The frequencytunable laser of claim 11 wherein the first and second laser cavities,the optical gain element, and the first and second phase modulators areformed on a photonic integrated circuit (PIC).
 23. The frequency tunablelaser of claim 11 wherein the first and second laser cavities and thefirst and second phase modulators are formed on a photonic integratedcircuit (PIC).
 24. The frequency tunable laser of claim 11 furthercomprising a digital-to-analog converter having an input that receivesdigital modulation information and having an output connected to anelectrical input of at least one of the first and second phasemodulator.
 25. The frequency tunable laser of claim 11 wherein the firstphase modulator is configured as an I-Q modulator that generates anoutput signal that rotates endlessly around the origin of the complexplane imparting in phase with time.
 26. The frequency tunable laser ofclaim 11 wherein modulation provided by the first and second phasemodulators cause certain ones of the first plurality of cavity modes toline up with certain ones of the second plurality of laser cavity modes.27. A frequency tunable laser comprising: a) at least two coupled lasercavities, each of the at least two coupled laser cavities having adifferent cavity length; b) a gain element positioned in a common areaof the at least two coupled laser cavities, the gain element amplifyinglight propagating in the at least two laser cavities; and c) at leasttwo phase modulators, a corresponding one of the at least two phasemodulators being positioned in a corresponding one of the at least twocoupled laser cavities, each of the at least two phase modulatorscomprising a separate electrical modulation input so that the at leasttwo phase modulators can separately modulate a phase of the light in itsrespective laser cavity and tune a frequency of the frequency tunablelaser.
 28. The frequency tunable laser of claim 27 wherein the at leasttwo coupled laser cavities comprise more than two coupled lasercavities.
 29. The frequency tunable laser of claim 27 wherein at leastone of the at least two phase modulators is an endless phase modulatorthat changes a phase of light in its respective laser cavity as afunction of time.
 30. The frequency tunable laser of claim 29 whereinthe endless phase modulator performs a substantially linear phase shiftof light in its respective laser cavity as a function of time.
 31. Thefrequency tunable laser of claim 27 further comprising at least twodigital-to-analog converters, a respective one of the at least twodigital-to-analog converters having an output that is coupled to arespective one of the at least two phase modulators.
 32. The frequencytunable laser of claim 27 wherein the at least two coupled lasercavities are formed on one or more closely coupled photonic integratedcircuits.
 33. The frequency tunable laser of claim 27 wherein the atleast two coupled laser cavities and the gain element are formed on oneor more closely coupled photonic integrated circuits.
 34. The frequencytunable laser of claim 33 wherein the least two coupled laser cavitiesand the gain element are formed of different materials.